Slots perhaps and torp launcher in some. Speed too. And oxygen containment 

1) support slots (other have gain 1 while some just the same)
 
2) increase of # of torp launcher (except for km which it loses is rear tubes on km ss5.5(6))

3) dp and speed

Sub 5.5 have more torpedo ammo.

Sub 5.5 is allmost imposseble to speedcap if its not a Km sub.

there are basic  REAL differences between some of the 5 and 5.5s..

ie KM  IXD2 has 2 torpers  XXI has one
  ie IJN 5.5 has only 1 torper

ie RN   M type has one torper  A type has 2 torpers



there are more support slots  in some as well so you need to know what you will need for each nation 


there used to be a spreadsheet for 5.5s in  the testing section..copy it ..it has a few whoopsies in it but generally helpful .


 



 

ITS YOUR LUCKY DAY i FOUND IT AND BUMPED IT UP ..DEMFUEHRER POST